A number of such systems are known and, in the United Kingdom a specific VHF frequency band with a 50 kHz channel spacing is allocated for such systems when used by utilities. FIG. 12 of the accompanying drawings schematically illustrates a typical arrangement for such a system.
A number of remote terminals RT communicate with fixed local data collectors or concentrators FDC using a radio data link. The data collected by the local concentrators is supplied to a central controller via a data network ISDN to which other local concentrators are connected, or via a radio link RL. Information collected at the central controller is then passed to customer billing and settlement departments.
A typical objective for such a system is to provide a 99% probability of radio coverage throughout the service area for a high proportion of the time. The remainder of the service area is covered by a fallback system in which meter reading data is collected manually. The network cost is dominated by the cost of the remote terminals which are required to be inexpensive. However the cost and complexity of the fixed collector infrastructure must also be minimized to enable a practical system roll-out.
An example of a multiple-point to point network, for use in the monitoring of consumer water supply meters, is described in U.S. Pat. No. 4,940,976 of Gastouniotis et al. The remote terminals of Gastouniotis are mounted on or adjacent to the casing of an existing water meter and include remote flow sensing apparatus in the form of a magnetic flux sensor which senses the varying magnetic field produced by the water meter's rotating magnet. A calculation unit converts the sensor signal into a binary count signal representative of the volume of water consumed. This binary count signal is transmitted to local fixed data collectors by a conventional low power frequency shift keyed (FSK) modulator and transmitter operating in the 72-76 MHz frequency band and having a power output of below 2 watts as required by United States FCC regulations for telemetry operations. Accumulated data from the fixed data unit is then transmitted to a central office by telephone line, CATV cable or half-duplex RF radio link.
Transmission of consumption data from remote terminal to local collector in the Gastouniotis system is in the form of a 128-bit burst (64 bits of unit identification header and 64 data bits) at 1600 bits/sec. A burst is transmitted each time the consumption meter is incremented and also periodically at a specified interval, such as once per day, determined by a timer within the remote terminal. The packet size and data rate are determined by transmission collision probability minimization such as to maximize the probability of at least one packet being successfully transmitted each day from each of 10,000 remote terminals to a single local connect. The reception area of the Gastouniotis local collectors is only about 7.8 square-kilometers (3 square miles) and the figure of 10,000 remote terminals is suggested as sufficient to achieve adequate coverage of the reception area.
The design of such networks must overcome a number of problems including the minimization of co-channel interference between neighboring remote terminals and the cost of the remote terminals. It is generally required that several thousand remote terminals be supported by each data collector. Key features in the design of a low cost remote terminal are low component count, ease of manufacture and a relaxed specification for frequency stability. As mentioned previously, the cost and complexity of the fixed collector infrastructure must also be minimized.